This invention relates generally to ink jet arrays including a plurality of ink jet channels wherein each channel includes a chamber, an inlet to the chamber, and orifice from the chamber, and transducer means coupled to the chamber for ejecting droplets of ink from the chamber as a function of the state of energization of the transducer means. More specifically, this invention relates to a method and apparatus for reducing cross talk in such ink jet arrays.
In liquid droplet ejecting systems of the drop-on-demand type, such as impulse ink jet printers, a piezoceramic transducer is used to cause expulsion of ink as droplets from a small nozzle or jet. An array of such jets is often utilized in high-speed, high-resolution printers where, as is well-known, the printing rate increases as the number of jets is increased, but decreases as the degree of resolution required is increased.
High speed, high resolution printing therefrom requires large members of jets in an array. For such large arrays it is important for various reasons to make the array as compact as possible by minimizing the spacing between jets. Such reasons include, but are not limited to, minimizing "over-travel" (i.e., the amount of time spent by jets beyond the printing region during a scan of the paper), and reducing the overall size and mass of the printhead to reduce the size and cost of the printer.
One suitable such printer is described in U.S. Pat. No. 4,459,601, issued July 10, 1984 to Stuart D. Howkins, assigned to the assignee of the present invention and incorporated herein by reference. In that arrangement, an ink jet apparatus of the demand or impulse type comprises a chamber and an orifice from which droplets of ink are ejected in response to the state of energization of a transducer which communicates with the chamber through a foot forming a movable wall. The transducer expands and contracts, in a direction having at least one component extending parallel with the direction of droplet ejection through the orifice, and is elongated in such direction, the electric field resulting from the energizing voltage being applied transverse to the axis of the elongation.
One problem common to all high-speed, high-resolution, drop-on-demand ink jet printers occurs because the jets of an array are spaced very close to one another. That is, the response of one jet in an array to its drive voltage can be affected by the simultaneously application of a drive voltage to another nearby jet. This can result in a phenomenon, known in the art as "mechanical cross talk", where pressure waves are transmitted through the solid structure in which the transducers are mounted, through the ink, and through the solid material in which the jets are formed, or in another phenomenon, known in the art as "electrical cross talk", where the relatively large drive voltages necessary for substantial displacement of transducers utilized in the prior art cause the simultaneous pulsing of an inappropriate transducer.
While the risk of electrical cross talk between ink jets in an array utilizing the teaching of U.S. Pat. No. 4,459,601 as discussed above will be minimized, the risk of mechanical cross talk remains. One approach for eliminating such mechanical cross talk in drop-on-demand ink jet printers, disclosed in U.S. Pat. No. 4,381,515, issued Apr. 26, 1983 to Lee L. Bain, induces electrical cross talk using passive elements which effectively neutralizes the mechanical cross talk. A resistor is first placed in series with each transducer and its associated electricl driver. Thereafter, one compensating resistor per channel is connected at one end between the series resistor and transducer of a respective channel and connected at the opposite end between the series resistor and transducer of an adjacent channel. As is apparent, such an arrangement would unnecessarily complicate the manufacturer of drop-on-demand ink jet printers having a multitude of channels, thereby also increasing their cost.
Another approach which alleviates the problem of mechanical cross talk is discussed in U.S. Pat. No. 4,439,780, issued Mar. 27, 1984 to Thomas W. DeYoung and Viacheslav B. Maltsev, assigned to the assignee of the present invention and incorporated herein by reference. In that arrangement, an ink jet array comprises a plurality of elongated transducers coupled to a plurality of ink jet chambers, the transducers being supported only at their longitudinal extremities. The support at the extremity remote from each chamber is provided such that no longitudinal motion along the axis of the longation of the transducers occurs, while the other extremity includes bearing means which substantially precludes lateral movement of the transducers transverse to their axis of elongation but permit the longitudinal movement thereof along the axis, thus minimizing mechanical cross talk between ink jets within the array. Other characteristic problems which are encountered in the implementation of high-speed, high-resolution impulse ink jet printers do not impact so much upon their operation, but indeed impact upon their fabrication. For example, the relatively small sizes of component parts used in densely packed arrays make them difficult to handle. An easily fabricated ink jet array is, therefore, preferred.
One earlier approach to the above-described problem is disclosed in U.S. Pat. No. 4,072,959, issued Feb. 7, 1978 to Rune Elmqvist. As discussed therein, a recorder operating with drops of liquid includes a comb-shaped piezoelectric transducer arranged such that individual teeth of the comb are associated respectively to a densely-packed array of ink jet chambers. The teeth, actually a series of elongated transducers, are energized by electrodes which apply a field transverse to the access of elongation. Each of the transducers is immersed in a common reservoir such that energization of one transducer associated with one chamber may produced cross-talk with respect to an adjacent chamber or chambers. In other words, there is no fluidic isolation from chamber to chamber between the various transducers or more accurately, segments of the common transducer. In addition to such cross talk, the construction shown in the Elmqvist patent poses a requirement for nonconductive ink.
As pointed out in U.S. Pat. No. 4,564,851, issued Jan. 14, 1986 to Kenth Nilsson and Jan Bolmgren, another problem with the design of the Elmqvist patent is that the clamping of the comb must occur with extreme precision. Since the thickness of the comb only amounts to a few tenths of a millimeter, a displacement between the spacer piece and the clamp of the same order of magnitude will lead to considerable changes in the flexural forces. The flexural lengths of the transducers in the two twist directions become different as a consequence, thus leading to imprecise writing. In accordance with the teachings of U.S. Pat. No. 4,564,851, therefore, individual transducers are formed by teeth of a comb-like piezoplate comprising a bilaminar plate of a layer of piezoceramic material and a carrier layer, the piezoceramic layer being provided with a reinforcing layer in the area of a spine of the comb shared by all teeth. Unlike the Elmqvist patent, a far simpler arrangement can be utilized as a clamp for the comb without making demands on the tolerances, since the precise length of the oscillatory parts of the transducer is defined by the front edge of the reinforcement layer.
In addition to the problems of cross talk addressed in the foregoing U.S. Pat. Nos. 4,072,959 and 4,562,851, it should be noted that both such patents incorporate transducers which utilize flexural motion to eject droplets of ink on demand. This flexural motion is less desireable than motion provided by transducers which are elongated in the direction of expansion and contraction since displacement can be made large simply by increasing the length of the transducer, with such increase in length not causing any decrease in the density of an array thusly formed. Moreover, large displacements can be achieved without applying large electrical voltages which could result in electrical cross talk. It is desireable, to limit the length of the transducer so as to limit the undesirable flexural motion which can result when the transducer becomes too long and thin, and achieve the proper length mode resonance, those of the Helmholtz frequency as described in the above reference U.S. Pat. No. 4,459,601.
Other prior art approaches which have sought to minimize mechanical cross talk mechanically decouple the transducer means. For example, U.S. Pat. No. 4,390,886, issued June 28, 1983 to S. Bertil Sultan, discloses an ink jet printer having a plurality of channels, each of the channels including a transducer mounted within its own housing. Rectangular recessed portions are formed in the housings to define slots between adjacent ink jet modules so as to reduce coupling of cross talk therebetween. Another approach, utilized in continuous stream ink jet printers, is disclosed in U.S. Pat. No. 4,095,232, issued June 13, 1978 to Charles L. Cha, and in U.S. Pat. Re. No. 31,358, reissued Aug. 23, 1983 to Cha et al. Those patents teach a stimulator which includes a pair of piezoelectric crystals vibrating in phase and which are mounted on opposite sides of a mounting plate which is coincident with a nodal plane. A reaction mass is positioned at the opposite end of the stimulator from a stimulation member which is coupled to the fluid. Neither of those patents, however, address the problems of mechanical cross talk in drop-on-demand ink jet printers.